Fail fast while initializing JedisSentinelPool

It's related to #726.

I modified JedisSentinelPool's behavior to fail fast on

* wrong master name is specified
* all sentinels are down or not reached

Actually I also think it should fail fast and let users know about it instead of keep retrying.

Please review and comment! Thanks!
"doesn't monitor master" also applies a kind of failure of connection. Right? To be honest, I don't have strong opinion on this because it can throws an exception to callers in any way. Maybe we can let the author make the final decision. If no one can judge, we can just follow your intention. :)